Chapter 4 Hydrogeological setting 26 4 Hydrogeological setting 4.1 Nubian Sandstone Aquifer System 4.1.1 Extension and boundaries The Nubian Sandstone Aquifer System (NSAS) is considered to be one of the most significant and potable groundwater basins in the world, as it is the only water resource for most of the areas sharing its valuable reserve. It extends over a vast area in Egypt, Libya, Sudan, and Chad (Figure 4.1). In spite of its vast area, it is considered as a broadly closed system, as it has natural boundaries to the east and southeast formed by the mountain ranges of the Nubian Shield and is bounded to the south and west by the mountainous outcrops of Kordofan Block, Ennedi, and Tibesti. To the southwestern part of the basin, the aquifer is bounded by the groundwater divide located between Ennedi and Tibesti Mountains. The natural northern boundary of the Nubian Sandstone Aquifer System is set to the so-called Saline-Freshwater Interface, whose location is considered spatially stable, although slight movement is believable (Thorweihe 1987; Thorweihe and Heinl, 1996, 1999, 2000). The NSAS has a complex structure, which precludes adequate identification of the hydraulic continuity between the various sub-basins. The hydraulic interconnection is emphasized between sub-basins within individual countries sharing the system as well as between the different basins forming the system at the regional level (Wycisk 1993, 1994; Klitzsch and Wycisk 1999). The NSAS consists of a thick sequence of coarse (refer to Chapter 2 for detailed geological outlines), clastic sediments of sandstone along with siltstone interbedded with impermeable shale and clay beds. The more claylike impermeable beds restrict the vertical movement of water. These beds of lower permeability, however, are lenticular and discontinuous at some locations (Idris and Nour 1990; Thorweihe and Heinl 2002). Chapter 4 Hydrogeological setting 27 4.1.2 Subunits of the aquifer system The area of the Nubian Sandstone Aquifer System is composed of different water bearing strata (CEDARE 2002; Foster and Loucks 2006) laterally and/or vertically interconnected. It can be differentiated into two major reservoirs (Figure 4.1). The oldest and the most extended reservoir is the Nubian Aquifer System (NAS). It is largely prevailing unconfined conditions and includes a number of basins that are hydraulically connected. The unconfined part of the Nubian aquifers includes the most important groundwater potential of the whole basin (Bakhbakhi 2006). The other reservoir includes parts of Libya and Egypt. It is referred to as the Post Nubian Aquifer System (PNAS). The NAS underlies almost all the area of Egypt, Eastern Libya, Northern Sudan, and Northern Chad. The NAS comprises the Paleozoic and the Mesozoic deposits and overlies the Pre-Cambrian basement complex. The PNAS occurs generally to the north of the 25th latitude overlying the NAS in the north of the Western Desert of Egypt and Northeastern Libya (Bakhbakhi 2006). The PNAS comprises the Tertiary continental deposits in Libya and Egypt and the Tertiary (Upper Cretaceous-Eocene) shale and carbonate complex that confine the system at the topographic depressions in Egypt. A low permeability layer belonging to the Upper Cretaceous and the lower Tertiary sediments separates the two reservoir systems (CEDARE 2002; Bakhbakhi 2006). Groundwater mining of the reserves of the Nubian Sandstone Aquifer System is presently taking place and increasing yearly. In the past 40 years over 40 billion cubic meters (40x109 m3) of groundwater has been exploited from the system in Egypt and Libya (Margat et al. 2006). This has provoked the groundwater level to decline progressively. Consequently, over 35% of the free-flowing wells and springs in many locations had to be deepened or substituted with deeper wells. The area of the Nubian Sandstone Aquifer System of the Eastern Sahara is about 2.35 km2. A great part of this area lies on the Egyptian side, as it possesses about 880,000 km2 distributed nearly all over the country (CEDARE 2001). The remaining area is contributed by the other sharing countries as shown in Table 4.1. Chapter 4 Hydrogeological setting 28 Figure 4.1: Regional hydrogeological overview of the Nubian Sandstone Aquifer System. Compiled from: CEDARE (2002), Salem and Pallas (2004), Bakhbakhi (2006). Chapter 4 Hydrogeological setting 29 Groundwater storage of the NSAS in the Eastern Sahara is huge and represents the largest fresh groundwater reserve in the Arab world or maybe in the whole world (CEDARE 2001, 2002). This issue has been published frequently. The most known studies are from Ambroggi (1966), who estimated the total groundwater volume of the Sahara at 15,000 km3, and from Gischler (1976), who considered it to be at least 60,000 km3. Thorweihe and Heinl (1996) estimated the groundwater volume of the Nubian Sandstone Aquifer System 150,000 km3, with a very large amount of water largely exceeding the previous estimates. CEDARE/IFAD (2002) in a program for the development of a regional strategy for the utilization of the Nubian Sandstone Aquifer System has estimated the fresh groundwater volume within the system at 372,950 km3 (Table 4.1). Based on the most updated database and the GIS technique, the groundwater resources of the Nubian Sandstone Aquifer System were calculated by the author using the modeled saturated volume and the calibrated hydraulic parameters of the system (refer to Chapter 6). Since it is economically unreasonable and infrastructurally impossible to obtain groundwater from great depths over broad areas of the aquifer system, the total volume of the groundwater resources is to some extent of academic interest. For the interpretation of groundwater ages based on radiocarbon analyses, however, it is very important to have an idea of the groundwater's mean residence time, which is dependent on water volume and discharge rate (Figure 4.5). Table 4.1. Area, freshwater storage, and present extraction rates of the Nubian Sandstone Aquifer System. Area Freshwater storage1 Present extraction (1000 km2) (km3) (km3/year) Country PNAS NAS Total PNAS NAS Total PNAS NAS Total Egypt 569 311 880 102,417 52,299 154,716 0.306 0.2 0.506 Libya 350 300 650 11,240 125,309 136,549 0.264 0.567 0.831 Sudan - 750 750 - 47,807 47,807 - 0.84 0.84 Chad - 70 70 - 33,878 33,878 - 0.0 0.0 Total 919 1430 2,350 113,657 259,293 372,950 0.57 1.607 2.177 1 Assuming a storativity of 10-4 of the confined part of the aquifer and 7% effective porosity of the unconfined part. Source: CEDARE/IFAD (2002). Programme for the Development of a Regional Strategy for the Utilization of the Nubian Sandstone Aquifer System. Chapter 4 Hydrogeological setting 30 4.1.3 Basins of the NSAS Regional faults and uplifts divide the entire Nubian Sandstone Aquifer System into four basins (Ezzat 1974; RIGW 1988, 1993, 1999; FAO 1997; Attia 1998; MPWWR 1999; Thorweihe and Heinl 2002). Two of them are major basins and of high significance, and these are the Kufra Basin of eastern Libya, northeastern Chad, the most northwestern strip of Sudan, and the Dakhla Basin of the Western Desert of Egypt. The other two subunits are the Upper Nile platform and northern Sudan Platform; they are of minor importance in relation to the Kufra and Dakhla basins, as the thickness of their sediments ranges from little wedges to a few hundred meters (Klitzsch 1994; Klitzsch and Wycisk 1999). The Kufra and Dakhla basins are filled by fluvial Paleozoic and Mesozoic sandstone forming an enormous groundwater-bearing formation with a thickness reaching 3,500 m in the northern part of the system in Egypt and about 4,500 m in the northern (Klitzsch and Squyres 1990) part of the Kufra Basin in Libya (Figure 4.2). On the other hand, continental sediments of a few hundred meters’ thickness predominately cover the Upper Nile and northern Sudan platforms (Heinl and Brinkmann 1989). These basins are discussed briefly as follows (Attia 1998; RIGW 1999): 4.1.3.1 The Northwestern Basin It lies northwest of the Cairo-Bahariya Uplift that separates it from the Dakhla Basin to the south of it (Figure 4.2). Only the southernmost strip of that unit stretches to the south of the Saline-Freshwater Interface that outlines the northern boundary of the Nubian Sandstone Aquifer System. It is composed of marine shale and limestone that have thickness ranges from 3,000-5,000 m, and in its central part these marine sediments are overlain by Miocene continental sandstone, forming a phreatic aquifer with very good hydraulic conductivity (Hantar 1990). 4.1.3.2 The Dakhla Basin It is the largest and most important unit in Egypt. It consists of fine to coarse grained continental sandstone of Lower Cretaceous with good permeability and thickness reaching 3,500 m. Two beds of the Silurian, low permeable sediments, which are of marine origin, occur mainly as marine shale and intercalate the sandstone sequence (Hermina 1990, Wycisk 1993). These two beds are of a few tens of meters in thickness and thicken to the north. The aquifer system is unconfined to the south of the basin, where the sandstone outcrops, while to the north, where the sandstone is overlain by the impermeable marine shale and clay, the aquifer reveals confined conditions (Hermina 1990; Klitzsch and Schandelmeier 1990; Klitzsch and Wycisk 1999). Actually, all the Egyptian oases and almost all the newly constructed development areas lie in this basin. Chapter 4 Hydrogeological setting 31 Figure 4.2: Locality map showing the thickness, major uplifts, and the structural units of the NSAS. Locations of uplifts adapted from thickness after Klitzsch (1983), Hissene (1986), Hesse et al.